Altitude of the upper boundary of AAR based on observations of ion beams in inverted-V structures: A case study

Outflowing ion beams forming four successive inverted-V structures in the energy-time spectrograms of H⁺, He⁺, and O⁺ were observed at an altitude of 3.4 R_E by Cluster satellites travelling above the auroral acceleration region (AAR) in the southern hemisphere on February 14, 2001. Energization by negative U-shaped potential structures in the AAR is believed to be responsible for the formation of these outflowing ion inverted-V structures. Thus, utilizing the different motion properties of the three ion species, the altitude of the upper boundary of the AAR is estimated to be ~11100 km. Moreover, based on multi-satellite observations, each ofthese U-shaped potential structures involved in this event crosses the latitud in aldirection at ~0.4°-1° invariant latitude (ILAT), moving poleward at an average speed of ~0.2° ILAT per minute, before disappearing at ~71.5° ILAT.